Non-impact printers often employ apparatus to form latent electrostatic images on a suitable surface, which surface with its latent image electrostatic charge attracts toner from a developer station and subsequently transports the toner image to a transfer station where the toner is transferred to paper. In accordance with normal conventional techniques, the toner particles themselves are inductively or triboelectrically charged to a potential opposite to that of the latent image and close to the potential of the background of the image so that, ideally, the toner will adhere electrostatically to the image and not to the background surface. Such techniques are common in xerography and in electrographic printing as shown by U.S. Pat. No. 4,638,339 and U.S. Pat. No. 4,642,661, both of which are assigned to the assignee of the present invention.
A commonly used technique for transferring the image to the paper is to expose the back side of the paper to the field of a corona the polarity of which is opposite to that of the toner that adheres to the image with transfer of the image taking place when the paper is in contact with the image surface so that the charge on the paper attracts the toned image to the paper which thereafter is fixed to the paper as, for example, is described in the aforementioned U.S. Pat. No. 4,642,661.
Under ideal conditions the development process will faithfully reproduce the electrostatic charge pattern of the image and the transfer of the tone image would be relatively simple, the objective being to transfer as much of the toner as is possible while maintaining the quality of the image.
Such conventional techniques of developing the image have resulted in the identification of two basic problems. One such problem is presented by creation of a background that is less than "clear" because some of the toner particles adhere to the surface on which the latent image is formed--even in areas where no electrostatic image is present. That toner ends up being transferred to the paper as background clutter so as to detract from the quality of the printed page. A second problem results from the action of the charged toner particles themselves which tend to be attracted to the edges of the latent electrostatic image thereby creating unequal image density and even some loss of edge definition. Attempts have been made to control the image signal field strength - namely, the white-black field difference--such that the background collection of spurious toner particles is limited; but unfortunately such a technique generally limits the density of the image itself.